This invention pertains to the art of electrical control systems and more particularly to a heat sensing device for sensing molten metal penetration through the refractory lining of an induction furnace.
The invention is particularly applicable to a sensing mat interposed between a refractory lining of a furnace and the heating coils thereof so that in the event of molten metal penetration through the refractory lining, the system can provide a warning and power application to the coils can be interrupted. However, it will be appreciated to those skilled in the art that the invention could be adapted for use in other environments besides induction furnaces, as for example, where undesired increases in temperatures are indicative of a dangerous problem and where energy application should be interrupted.
The coils of an induction furnace are spaced and insulated from the molten metal of the furnace hearth by a refractory lining. Such linings are exposed to severe stresses mechanically, chemically and thermally, and cracks in the lining will regularly occur that have to be repaired to preclude metal penetration to the coil, which with the power levels employed in such furnaces, is a dangerous situation. Ground detector systems have been most commonly employed to identify the occurrence of dangerous metal penetration through the lining. Such systems are well known and in use. The subject invention is intended to supplement the present ground detection systems and thereby enhance warning control.
In order to detect molten metal penetration in the furnace lining, the normal ground detector system has required metal penetration to be in electrical contact with the power coil. When the system is installed correctly and maintained well, on most melting systems it has proven to work satisfactorily.
The typical ground detection system has two main problems. The first is that it relies heavily on the integrity of the electrical connection between the molten metal in the furnace hearth and ground spider wires embedded in the furnace refractory floor. If the wires are not properly installed or if the floor is refractory patched, electrical continuity could be broken resulting in the inability to detect a metal penetration to the coil. If the furnace lining is made from a precast, prefired crucible shape, the spider wires will never be in electrical contact with the molten metal thereby making the system unable to detect a metal penetration to the coil.
The second problem is that it relies on actually having the metal penetration in electrical contact with the coil. Since coils now run at higher voltages, electrical insulation on the coil is critical with higher insulation requirements. In order for the ground detector to detect this penetration, the metal must first burn through all layers of the electrical insulation to contact the coil. If the metal penetration reaches the coil there will be a production down time for the furnace owner associated with repair of the furnace coil. The down time could range anywhere from a major repair (days to weeks) if severe damage occurs, or a minor repair if the penetration is just a small metal fin (hours).
In U.S. Pat. No. 5,319,679 to Hopf, a ceramic fiber foil mat encasing electrodes is used for electrical resistance measurement of the ceramic furnace lining between two electrodes. In this system, one layer of the sensing mat rests directly on the lining to emulate the lining properties of electrical conductivity and thermal conductivity. Thus, the electrical resistance of the ceramic fiber can be related to the resistance of the furnace lining.
The problem with such a system as Hopf is that it relies on an assured engagement between the sensing mat and the refractory lining. In the event such engagement is lost, reliability of the system becomes correspondingly diminished.
The present invention contemplates a new and improved control apparatus which provides an enhanced warning system as a backup to the normal ground detector system of an induction furnace. The system will detect metal penetration in the furnace lining to a high degree of reliability before it can actually pierce through the lining and the detector to actually contact the coil. The subject system's reliability is high because it does not rely on the actual contact between the conventional ground detect embedded spider wires and molten insulation; and the subject system does not rely on the metal penetration burning through the coil electrical insulation.